Long Term Evolution (LTE) projects are the evolution of 3G. LTE is not a 4G technology which is commonly misunderstood by people, and instead, it is a transition between 3G and 4G technologies. LTE is a 3.9G global standard, and uses OFDM and MIMO as an unique standard of its wireless network evolution, which improves and enhances the 3G air access technology. This technology with the OFDM/FDMA as a core technology can be treated as a “quasi-4G” technology. In a spectral bandwidth of 20 MHz, it can provide a peak rate of 100 Mbit/s in the downlink and a peak rate of 50 Mbit/s in the uplink, which improves the performance for users at a cell edge, enhances a cell capacity and reduces system latency.
The performance of the wireless system depends on a time-varying condition of a wireless link, which means that, for example, Block Error Ratio (BLER), throughput and delay are not constant. In order to deal with the changing condition of the wireless link and provide a reliable QOS, it is necessary to select an appropriate scheduling strategy. A processing mechanism of achieving dynamic adjustment is link adaptation. Generalized link adaptation includes inner loop link adaptation and outer loop link adaptation, HARQ and resource scheduling for matching channels etc.
The Inner Loop Link Adaption (ILLA) is mainly based on a Signal to Interference ratio (SINR). For this approach, a reasonable SINR threshold is set for each supported modulation and coding scheme, which requires consistency with the UE capability. Specifically, a terminal provides a CQI to a base station and the base station selects a MCS based on the CQI which is fed back.
The purpose of the Outer Loop Link Adaption (OLLA) is to maintain a packet loss rate to be above a fixed level by dynamic adaptive thresholds, except that differences between these thresholds remain the same. The base station may assign a specific offset value to a terminal, which can be used to adjust a predicted SINR value.
Since the transmission power in the LTE downlink is constant, the LTE employs different link adaptation technologies in order to accommodate rapid changes in the radio channel. Firstly, the Modulation and Coding Scheme (MCS) adapts to the channel quality at some frequency intervals based on feedback from a User Equipment (UE). Secondly, an evolved base station (eNodeB) has a capability of performing Frequency Domain Packet Scheduling (FDPS) to allocate the most suitable resources to the user. The purpose of Link Adaptation (LA) is to process the resulting feedback information from the terminal and then to select an appropriate MCS based on the information on a location of allocation in the frequency domain.
In Long Term Evolution (LTE) and Long Term Evolution Advanced (LTE-A) systems, the link adaptation adopts a method of combining inner loop link adaptation and outer loop adaptation. The ILLA is firstly responsible for selecting an appropriate MCS for the UE. This selection is based on a mapping relationship between a measured SINR and an allocated optimum MCS. The ILLA does not always adapt well to the channel (for example, rapid channel change) for a variety of reasons. Therefore, the function of the OLLA is also necessary. The purpose of the OLLA is to achieve a target BLER by adjusting the MCS selection. For example, the target BLER=0.1 in the LTE, and the base station can determine a current BLER by statistically analyzing HARQ ACKs fed back by the UE. Therefore, this method is based on Hybrid Automatic Repeat Request (HARQ)-ACK feedback information for first HARQ transmission.
In the LTE and LTE-A, the HARQ is a scheme of combining the ARQ and the FEC to retransmit only data packets with errors. The HARQ technology can well compensate for the influences of time variation and multipath fading of the wireless mobile channel on signal transmission, and has become one of indispensable key technologies in the system. The HARQ uses an incremental redundancy retransmission mechanism, and for each transmitted data packet, a complementary deletion manner is adopted. Various data packets can not only be decoded individually, but also can be combined into a coded packet with more redundant information and decoded as a whole. The system can support a plurality of HARQ processes simultaneously, and one HARQ process corresponds to one transport block. On the base station side, a CRC is firstly added to one transport block, which is then coded and modulated to form a stream of code words. One stream of code words is mapped to one or more layers, and is then mapped to a plurality of OFDMA sub-carriers, which are subsequently processed and are transmitted to a terminal through an air interface. On the terminal side, it is firstly judged whether the received stream of code words is first transmitted data or retransmitted data of the transport block. If it is first transmitted data, the stream of code words is directly decoded, if it is decoded correctly, ACK is generated, and if it is decoded wrongly, NACK is generated. Otherwise, data of the last code word and data of the currently received code word in an HARQ buffer are combined, and are then decoded. If it is decoded correctly, ACK is generated, and if it is decoded wrongly, NACK is generated. The generated ACK or NACK is referred to as HARQ-ACK acknowledgement information, and the terminal feeds back the acknowledgement information to the base station. On the base station side, if the acknowledgment information is ACK, it indicates that the transport block is transmitted successfully. If the acknowledgment information is NACK, it indicates that the transport block fails to be transmitted and a retransmission packet is required to be transmitted.
In the LTE and LTE-A, for control signaling required to be transmitted in the uplink, there are ACK/NACK and three forms which reflects downlink physical Channel State Information (CSI), which are Channels quality indication (CQI), a Pre-coding Matrix Indicator (PMI), and a Rank Indicator (RI).
The CQI plays a key role in the link adaptation process, and is a message transmitted by the UE to the eNodeB for describing a current downlink channel quality of the UE. The UE may measure a reference symbol transmitted by the base station, and then calculate the CQI.
The CQI is an index used to evaluate whether the downlink channel quality is good or bad. In the 36-213 protocol, the CQI is represented using an integer value within a range of 0 to 15, which represents different CQI levels respectively. Different CQIs correspond to respective MCSs, as shown in Table 1. The selection of the CQI level should follow the following criteria:
the selected CQI level should enable a block error rate of a PDSCH transport block corresponding to the CQI under a corresponding MCS not to exceed 0.1.
Based on a non-limited detection interval in the frequency domain and the time domain, the UE will obtain the highest CQI value, corresponding to each of the maximum CQI values transmitted in an uplink subframe n, the CQI serial numbers range from 1 to 15, and satisfy the following condition: an error rate BLER of a single PDSCH transport block is not more than 0.1 when the transport block is received, if CQI serial number 1 does not satisfy the condition, the CQI serial number is 0. The PDSCH transport block contains combined information, i.e. a modulation scheme and a transport block size, which corresponds to a CQI serial number and a set of occupied downlink physical resource blocks, i.e. CQI reference resources. Herein, the highest CQI value means a maximum CQI value which ensures that the BLER is not more than 0.1, this is beneficial for controlling the resource allocation. In general, the smaller the CQI value is, the more the resources are occupied, and the better the performance of the BLER is. Herein, the BLER is the error rate of the transport block, and the BLER is equal to the number of correctly transmitted TBs divided by the total number of transmitted TBs.
For the combined information having the transport block size and the modulation scheme which corresponds to a CQI sequence number, according to the related transport block size, the combined information for PDSCH transmission in the CQI reference resources can be notified using signaling, and additionally:
the modulation scheme is represented by the CQI serial number and uses the combined information including the transport block size and the modulation scheme in the reference resources, an effective channel coding rate generated by it is the most likely close effective channel coding rate which can be represented by the CQI serial number. When there is more than one piece of combined information and they can all generate equally close effective channel coding rates represented by the CQI serial number, combined information with the smallest transport block size is used.
Each CQI serial number corresponds to a modulation scheme and a transport block size. A correspondence relationship between transport block size and NPRB is shown in Table 1. A coding rate can be calculated according to the transport block size and a size of the NPRB.
TABLE 14-bit CQI tableCQI indexmodulationcode rate × 1024efficiency0out of range1QPSK780.15232QPSK1200.23443QPSK1930.37704QPSK3080.60165QPSK4490.87706QPSK6021.1758716QAM3781.4766816QAM4901.9141916QAM6162.40631064QAM4662.73051164QAM5673.32231264QAM6663.90231364QAM7724.52341464QAM8735.11521564QAM9485.5547
There are many CQI definitions in the LTE, and the CQI can be divided according to different principles:
according to a measurement bandwidth, the CQI is divided into a wideband CQI and a subband CQI;
the wideband CQI refers to channel state indications of all the subbands, and CQI information of a subband set S is obtained;
the subband CQI refers to CQI information for each subband. In the LTE, according to different system bandwidths, RBs corresponding to an effective bandwidth are divided into a number of RB groups, and each RB group is referred to as a subband.
The subband CQI can also be divided into an all subband CQI and a Best M CQI. For the all subband CQI, CQI information of all subbands is transmitted; and for the Best M CQI, M subbands are selected from the subband set S and CQI information of the M subbands is transmitted while location information of the M subbands is transmitted.
According to the number of code streams, the CQI is divided into a single-stream CQI and a dual-stream CQI.
The single-stream CQI is applied in single-antenna transmitting port 0, port 5, transmit diversity, MU-MIMO, and closed-loop spatial multiplexing with RI=1, and at this time, the UE transmits CQI information of a single code stream.
The dual-stream CQI is applied in a closed-loop spatial multiplexing mode. For an open-loop spatial multiplexing mode, CQIs of two code streams are equal in the open-loop spatial multiplexing since channel state information is unknown and double-stream characteristics are equalized in the precoding.
According to a CQI representation method, the CQI is divided into an absolute value CQI and a differential CQI.
The absolute value CQI is a CQI index represented by 4 bits in Table 1; and the differential CQI is a CQI index represented by 2 bits or 3 bits. The differential CQI is further divided into a differential CQI of a second code stream with respect to a first code stream and a differential CQI of a subband CQI with respect to a subband CQI.
According to a CQI transmission scheme, the CQI is divided into a wideband CQI, a UE selected (subband CQI), and a high layer configured (subband CQI); the wideband CQI refers to CQI information of a subband set S; the UE selected (subband CQI) is a Best M CQI, CQI information of selected M subbands is fed back while positions of the M subbands are transmitted; and the high layer configured (subband CQI) is an all subband CQI, one piece of CQI information is fed back for each subband.
Both of the high layer configured and the UE selected are subband CQI feedback modes. In a non-periodic feedback mode, subband sizes defined by these two feedback modes are inconsistent. In the UE selected mode, a size of M is also defined.
In the LTE system, an ACK/NACK response message is transmitted on a Physical Uplink Control Channel (PUCCH) in a format 1/1a/1b (PUCCH format 1/1a/1b), and if a User Equipment (UE) needs to transmit uplink data, it is transmitted on a Physical Uplink Shared Channel (PUSCH). The feedback of the CQI/PMI and the RI may be periodic or non-periodic. A specific feedback is shown in Table 2.
TABLE 2Uplink physical channels corresponding to periodic feedbackand aperiodic feedbackPeriodic CQI reportingAperiodic CQI reportingScheduling modechannelchannelFrequencyPUCCHnon-selectiveFrequencyPUCCHPUSCHselective
Herein, for the CQI/PMI and the RI which are fed back periodically, if the UE does not need to transmit the uplink data, the CQI/PMI and the RI which are fed back periodically are transmitted on the PUCCH in a format 2/2a/2b (PUCCH format 2/2a/2b), and if the UE needs to transmit the uplink data, the CQI/PMI and the RI are transmitted on the PUSCH. For the CQI/PMI and the RI which are fed back aperiodically, they are only transmitted on the PUSCH.
The Release 8 standard of the Long Term Evolution (LTE for short) defines three downlink physical control channels as follows: a Physical Control Format Indicator Channel (PCFICH for short), a Physical Hybrid Automatic Retransmission Request Indicator Channel (PHICH for short), and a Physical Downlink Control Channel (PDCCH for short). Herein, the PDCCH is used for carrying Downlink Control Information (DCI for short), including: uplink and downlink scheduling information, and uplink power control information. The DCI formats are divided into the following: DCI format 0, DCI format 1, DCI format 1A, DCI format 1B, DCI format 1C, DCI format 1D, DCI format 2, DCI format 2A, DCI format 3 and DCI format 3A etc., herein the transmission mode 5 supporting the MU-MIMO utilizes downlink control information of the DCI format 1D, and a downlink power offset field δpower-offset in the DCI format 1D is used to indicate information of reducing power of a user by a half (i.e., −10 log 10(2)) in the MU-MIMO mode, since the MU-MIMO transmission mode 5 only supports MU-MIMO transmissions of two users. Through the downlink power offset field, the MU-MIMO transmission mode 5 can support dynamic switching between a SU-MIMO Mode and a MU-MIMO mode, but no matter whether in the SU-MIMO mode or the MU-MIMO mode, the DCI format only supports one stream transmission for one UE. Although the Release 8 of the LTE supports single-user transmission of at most two streams in the transmission mode 4, since switching between the transmission modes can only be semi-static, in the Release 8 of the LTE, dynamic switching between single-user multi-stream transmission and multi-user transmission cannot be achieved.
In the Release 9 of the LTE, in order to enhance downlink multi-antenna transmission, a transmission mode of dual-stream beamforming is introduced, which is defined as transmission mode 8, and DCI format 2B is added in the downlink control information to support such transmission mode. There is an identification bit of a Scrambling Identity (SCID for short) in the DCI format 2B to support two different scrambling sequences. The eNB can allocate the two scrambling sequences to different users, and multiplexing is performed for multiple users in the same resource. In addition, when only one transport block is enabled, a New Data Indication (NDI) bit corresponding to a disabled transport block is also used to indicate an antenna port during single-layer transmission.
As the mainstream standard of the fourth generation mobile communication, the Long Term Evolution Advanced (LTE-A) system is an evolved standard of the LTE, which supports a greater system bandwidth (up to 100 MHz) and is backward compatible with the existing standard of the LTE. In order to achieve higher average spectral efficiency of a cell and improve the coverage and throughput at a cell edge, on the basis of the existing LTE system, in the Rel-10 and Rel-11 releases, the LTE-A supports key technologies in the downlink such as SU/MU-MIMO dynamic switching of at most 8 antennas, Carrier Aggregation (CA), Coordinated Multi-point (COMP) transmission, Enhanced Inter-Cell Interference Coordination (eICIC), advanced Relay, enhanced PDCCH etc.
In addition, in Release 10 of the LTE, in order to further enhance multi-antenna transmission in the downlink, a new transmission mode of closed-loop spatial multiplexing is added, which is defined as transmission mode 9, and DCI format 2C is added in the downlink control information to support such transmission mode. This transmission mode can not only support single-user SU-MIMO, but also can support multi-user MU-MIMO, and can support dynamic switching therebetween. In addition, this transmission mode also supports 8-antenna transmission. This new transmission mode has determined to use a demodulation pilot (UE Specific Reference Signal (URS for short)) as a pilot for demodulation, and the UE can estimate a channel and interference on the pilot only by acquiring a location of the pilot.
Further, in Release 11 of the LTE, on the basis of the transmission mode 9, in order to further support the COMP transmission, transmission mode 10 is defined and DCI format 2D is added in the downlink control information to support this transmission mode.
In the R11 release, the UE is semi-statically configured through high-level signaling to receive PDSCH data transmission according to an indication of a PDCCH of a UE-specific search space based on one of the following transmission modes:
Transmission mode 1: Single antenna port; Port 0
Transmission mode 2: Transmit diversity
Transmission Mode 3: Open-loop spatial multiplexing
Transmission Mode 4: Closed-loop spatial multiplexing
Transmission Mode 5: Multi-user MIMO
Transmission mode 6: Closed-loop Rank=1 precoding
Transmission mode 7: single antenna port; port 5
Transmission mode 8: dual-stream transmission, that is, dual-stream beamforming
Transmission mode 9: up to 8 layer transmission
Transmission mode 10: Support up to 8 layer transmission of COMP
The Machine Type Communication (MTC for short) User Equipment (user device or terminal for short), which is also known as Machine to Machine (M2M for short) user communication device, is a main application form of the current Internet of Things. In recent years, due to the high spectral efficiency of the Long-Term Evolution (LTE for short) or Long-Term Evolution Advanced (LTE-Advance or LTE-A for short), more and more mobile operators select the LTE/LTE-A as an evolution direction of broadband wireless communication systems. Based on the MTC of the LTE/LTE-A, various types of data services will also be more attractive.
In the MTC application terminal, there is a class of terminals having a significant reduction in coverage performance due to limitations of their locations or their own characteristics. For example, MTC terminals such as intelligent meter reading are mostly installed in low-coverage performance environments such as a basement, and they mainly transmit small-packet data, require a low data rate, and can tolerate a large data transmission delay. Since such terminals require a low data rate, for a Physical Downlink Share Channel (PDSCH for short), a Physical Uplink Share Channel (PUSCH for short), a Physical Downlink Control Channel (PDCCH for short), a Physical Uplink Control Channel (PDCCH for short) etc., the coverage performance can be improved by transmitting the same information repeatedly.
Simultaneous retransmissions may occupy a large number of resources, and numbers of retransmissions corresponding to different requirements for coverage improvement are also different. If the transmission is always carried out according to the same number of retransmissions, when a channel condition changes, there will be a condition that the resources are wasted repeatedly or a retransmission number is not enough. Therefore, it is necessary to develop a set of retransmission level adjustment mechanisms.
On the one hand, in the traditional mobile communication systems, in the case of fast channel change, traffic data burst, interference data burst, cell switching, use of advanced receivers etc., the traditional link adaptation technology is inaccurate and not fast. For example, the OLLA implements outer-loop link adaptation based on the number of ACKs or NACKs in the first packet transmission. This method is semi-static (requiring tens to hundreds of milliseconds) and cannot work effectively in the above scenario.
On the other hand, in the conventional mobile communication systems, after the data is decoded, a 1-bit ACK/NACK is generated, the channel adaptive condition caused by data decoding cannot be fully utilized, and the feedback is limited seriously.